A hydrophilic coumarin-based polyester for ambient-temperature initiator-free 3D printing: Chemistry, rheology and interface formation

Radical-free photopolymerizable composites of hyaluronic acid and gelatin for tissue engineering

Photopolymerization is widely used in tissue engineering and biofabrication to pattern specific geometries and modulate physical properties. Commonly employed photochemistries rely on a photoinitiator that generates reactive free radicals when exposed to light, which can lead to cytotoxic effects due to interactions with biomolecules and cellular components. To mitigate these issues, we have developed hyaluronic acid and gelatin derivatives of umbelliferone, which can form dimers thanks to cyclobutene ring formation when exposed to long-wavelength UV light (365 nm). These reactions occur efficiently with reduced cytotoxicity and without the need of a photoinitiator. Ligation to the biopolymers was carried out with the incorporation of a triethylene glycol or n-octyl linker that enhances the conformational flexibility of umbelliferone and contributes to improve the rheological properties. By heat annealing these derivatives, we produced double network hydrogels with various compositions. We assessed their physical properties using rheological and uniaxial compression tests, evaluated their cytocompatibility by encapsulating articular chondrocytes, and conducted preliminary printability tests to determine their suitability for injection and extrusion-based biofabrication. The materials exhibited good cytocompatibility and cell adhesion, were successfully extrudable using a pneumatic bioprinter while maintaining cell viability and were compatible with regulatory-approved steam sterilization. Due to their unique properties, these umbelliferone derivatives are well-suited for tissue engineering and biofabrication applications, offering crucial advantages for future clinical translation. STATEMENT OF SIGNIFICANCE: This study introduces a method for preparing novel bioinks from coumarin derivatives of hyaluronic acid and gelatin, key biopolymers in tissue engineering. These derivatives enable photoinitiator-free photocrosslinking without generating free radicals, thereby reducing cytotoxic risks and facilitating easier clinical translation compared to existing approaches. They are compatible with steam sterilization and show promise for extrusion-based techniques like bioprinting and injectability. Through mechanical characterization and biological assessments, the interactions between the biopolymers at different ratios and their effect on encapsulated cells were studied, providing insights for optimizing future tissue engineering applications.

Synergistic Effect of Physical and Chemical Cross-Linkers Enhances Shape Fidelity and Mechanical Properties of 3D Printable Low-Modulus Polyesters

Three-dimensional (3D) printing is becoming increasingly prevalent in tissue engineering, driving the demand for low-modulus, high-performance, biodegradable, and biocompatible polymers. Extrusion-based direct-write (EDW) 3D printing enables printing and customization of low-modulus materials, ranging from cell-free printing to cell-laden bioinks that closely resemble natural tissue. While EDW holds promise, the requirement for soft materials with excellent printability and shape fidelity postprinting remains unmet. The development of new synthetic materials for 3D printing applications has been relatively slow, and only a small polymer library is available for tissue engineering applications. Furthermore, most of these polymers require high temperature (FDM) or additives and solvents (DLP/SLA) to enable printability. In this study, we present low-modulus 3D printable polyester inks that enable low-temperature printing without the need for solvents or additives. To maintain shape fidelity, we incorporate physical and chemical cross-linkers. These 3D printable polyester inks contain pendant amide groups as the physical cross-linker and coumarin pendant groups as the photochemical cross-linker. Molecular dynamics simulations further confirm the presence of physical interactions between different pendants, including hydrogen bonding and hydrophobic interactions. The combination of the two types of cross-linkers enhances the zero-shear viscosity and hence provides good printability and shape fidelity.

Current state of knowledge on intelligent-response biological and other macromolecular hydrogels in biomedical engineering: A review

Because intelligent hydrogels have good biocompatibility, a rapid response, and good degradability as well as a stimulus response mode that is rich, hydrophilic, and similar to the softness and elasticity of living tissue, they have received widespread attention and are widely used in biomedical engineering. In this article, we conduct a systematic review of the use of smart hydrogels in biomedical engineering. First, we introduce the properties and applications of hydrogels and compare the similarities and differences between traditional hydrogels and smart hydrogels. Secondly, we summarize the intelligent hydrogel types, the mechanisms of action used by different hydrogels, and the materials for preparing different types of hydrogels, such as the materials for the preparation of temperature-responsive hydrogels, which mainly include gelatin, carrageenan, agarose, amylose, etc.; summarize the morphologies of different hydrogels, such as films, fibers and microspheres; and summarize the application of smart hydrogels in biomedical engineering, such as for the delivery of proteins, antibiotics, deoxyribonucleic acid, etc. Finally, we summarize the shortcomings of current research and present future prospects for smart hydrogels. The purpose of this paper is to provide researchers engaged in related fields with a systematic review of the application of intelligent hydrogels in biomedical engineering. We hope that they will get some inspiration from this work to provide new directions for the development of related fields.

A Review of Multi-Material 3D Printing of Functional Materials via Vat Photopolymerization

Additive manufacturing or 3D printing of materials is a prominent process technology which involves the fabrication of materials layer-by-layer or point-by-point in a subsequent manner. With recent advancements in additive manufacturing, the technology has excited a great potential for extension of simple designs to complex multi-material geometries. Vat photopolymerization is a subdivision of additive manufacturing which possesses many attractive features, including excellent printing resolution, high dimensional accuracy, low-cost manufacturing, and the ability to spatially control the material properties. However, the technology is currently limited by design strategies, material chemistries, and equipment limitations. This review aims to provide readers with a comprehensive comparison of different additive manufacturing technologies along with detailed knowledge on advances in multi-material vat photopolymerization technologies. Furthermore, we describe popular material chemistries both from the past and more recently, along with future prospects to address the material-related limitations of vat photopolymerization. Examples of the impressive multi-material capabilities inspired by nature which are applicable today in multiple areas of life are briefly presented in the applications section. Finally, we describe our point of view on the future prospects of 3D printed multi-material structures as well as on the way forward towards promising further advancements in vat photopolymerization.

Recent Advances in Functional Polymers Containing Coumarin Chromophores

Natural and synthetic coumarin derivatives have gained increased attention in the design of functional polymers and polymer networks due to their unique optical, biological, and photochemical properties. This review provides a comprehensive overview over recent developments in macromolecular architecture and mainly covers examples from the literature published from 2004 to 2020. Along with a discussion on coumarin and its photochemical properties, we focus on polymers containing coumarin as a nonreactive moiety as well as polymer systems exploiting the dimerization and/or reversible nature of the [2πs + 2πs] cycloaddition reaction. Coumarin moieties undergo a reversible [2πs + 2πs] cycloaddition reaction upon irradiation with specific wavelengths in the UV region, which is applied to impart intrinsic healability, shape-memory, and reversible properties into polymers. In addition, coumarin chromophores are able to dimerize under the exposure to direct sunlight, which is a promising route for the synthesis and cross-linking of polymer systems under "green" and environment-friendly conditions. Along with the chemistry and design of coumarin functional polymers, we highlight various future application fields of coumarin containing polymers involving tissue engineering, drug delivery systems, soft robotics, or 4D printing applications.

A photo-reversible crosslinking resin for additive manufacturing: reversibility and performance

Improving the adhesion between layers and achieving the recycling of resins are challenges in additive manufacturing (AM) technology. In this work, a new type of photo-reversible crosslinking resin based on polyvinyl alcohol (PVA) and coumarin (HMC) was prepared via grafting reaction. The critical idea was to create a coumarin based photo-reversible crosslinking resin by carefully tailoring the photo-crosslinking time and temperature, so that the resin could be extruded through the nozzle and then maintain the proper shape during UV-curing. Photo-reversible crosslinking of AM resin was realized without the use of monomers, photo-initiators or propagating. A reasonable irradiation time with 354 nm (crosslink) or 254 nm UV light (cleavage) of 10 min was critical for photo-reversible crosslinking of PVA-g-HMC at 120 °C. An important result of this work was that the developed photo-reversible crosslinked resin could be reused and the printed resin exhibits excellent adhesion properties, thermal conductivity and oxygen barrier performance.

Thread Size and Polymer Composition of 3D Printed and Electrospun Wound Dressings Affect Wound Healing Outcomes in an Excisional Wound Rat Model

Thread size and polymer composition are critical properties to consider for achieving a positive healing outcome with a wound dressing. Three-dimensional (3D) printed scaffolds and electrospun mats both offer distinct advantages as replaceable wound dressings. This research aims to determine if the thread size and polymer compositions of the scaffolds affect skin wound healing outcomes, an aspect that has not been adequately explored. Using a modular polymer platform, four polyester direct-write 3D printed scaffolds and electrospun mats were fabricated into wound dressings. The dressings were applied to splinted, full thickness skin wounds in an excisional wound rat model and evaluated against control wounds to which no dressing was applied. Wound closure rates and reduction of the wound bed width were not affected by the thread size or polymer composition. However, epidermal thickness was larger in wounds treated with electrospun dressings and was slightly affected by the polymer composition. Two of the four tested polymer compositions lead to delayed reorganization of granulation tissues. Moreover, enhanced angiogenesis was seen in wounds treated with 3D printed dressings compared to those treated with electrospun dressings. The results from this study can be used to inform the choice of dressing architecture and polymer compositions to achieve positive wound healing outcomes.

Toxicity and photosensitizing assessment of gelatin methacryloyl-based hydrogels photoinitiated with lithium phenyl-2,4,6-trimethylbenzoylphosphinate in human primary renal proximal tubule epithelial cells

Gelatin methacryloyl (GelMA) and lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) photoinitiator are commonly used in combination to produce a photosensitive polymer but there are concerns that must be addressed: the presence of unreacted monomer is well known to be cytotoxic, and lithium salts are known to cause acute kidney injury. In this study, acellular 10% GelMA hydrogels cross-linked with different LAP concentrations and cross-linking illumination times were evaluated for their cytotoxicity, photosensitizing potential, and elastic moduli. Alamar Blue and CyQuant Direct Cell viability assays were performed on human primary renal proximal tubule epithelial cells (hRPTECs) exposed to extracts of each formulation. UV exposure during cross-linking was not found to affect extract cytotoxicity in either assay. LAP concentration did not affect extract cytotoxicity as determined by the Alamar Blue assay but reduced hRPTEC viability in the CyQuant Direct cell assay. Photocatalytic activity of formulation extracts toward NADH oxidation was used as a screening method for photosensitizing potential; longer UV exposure durations yielded extracts with less photocatalytic activity. Finally, elastic moduli determined using nanoindentation was found to plateau to approximately 20-25 kPa after exposure to 342 mJ/cm2 at 2.87 mW of UV-A exposure regardless of LAP concentration. LAP at concentrations commonly used in bioprinting (<0.5% w/w) was not found to be cytotoxic although the differences in cytotoxicity evaluation determined from the two viability assays imply cell membrane damage and should be investigated further. Complete cross-linking of all formulations decreased photocatalytic activity while maintaining predictable final elastic moduli.

Role of pendant side-chain length in determining polymer 3D printability

The effect of polymer side chain on extrusion-based direct-write 3D printing and rheology is examined. Longer side chain length improves printability at ambient temperatures.